So called ad-hoc networks within Wireless Local Area Network (WLAN) systems are infrastructure-less networks. Nodes (e.g. subscribers or users) in an ad-hoc network are usually mobile, have a short-range wireless access and can join and leave the network at any time. Due to a limited range of applied wireless technologies, multiple hops are usually needed for communication.
Since there is no infrastructure within the system, the nodes themselves have to forward packets for other nodes, and hence have to act as both hosts and routers. In an ad-hoc network, the nodes are also responsible for network configuration and maintenance. Several routing protocols have been proposed for this kind of environment, usually modifications of the existing routing protocols for fixed networks. Examples of the most advanced ones are AODV (Ad hoc On Demand Distance Vector) and DSR (Dynamic Source Routing).
The AODV specification suggests either link layer information or usage of certain messages, for example the so called HELLO message, for neighbourhood detection. Since not all kinds of hardware provides support for adequate link layer notifications, such messages are used in many implementations. The specification defines that whenever a node A receives such a message from node B, node A includes node B into the routing table, and can eventually use node B for route establishment. If node A does not receive a certain number of consecutive HELLO messages from node B (each node broadcasts HELLO message periodically), it removes node B from the routing table.
There is a possibility that node A will receive HELLO messages from node B without node B receiving user packets from node A. This is due to the fact that certain kinds of hardware treat broadcast and unicast packets differently, and hence the range of respective packets is different. This causes a major problem for AODV performance, since neighbouring nodes can be falsely detected and routes established although it is not possible to send packets over them.
Another problem is that even when it is possible to send user packets to the neighbouring node, the quality of the link between the neighbours can be very poor and will cause errors, retransmissions, lower throughput and eventually route breakage. In this case, it would be better to establish a multi-hop route with high quality links than a 1-hop route with a low quality link.